Phototube



March '22, 1938.

M. c. TEVES 2,112,124

PHOTOTUBE Filed June 2'7, 1936 INVENTQR MARTE N CORNELIS TEVES fi l d 41 ATTORNEY Patented Mar. 22, 1938 PATIENT orries PHOTOTUBE Marten Cornelis Teves, Eindhoven, Netherlands,

assignor to N. V. Philips Gloeilampenfabrieken, Eindhoven, Netherlands Application June 27, 1936, Serial No. 87,625

In Germany July 29, 1935 4 Claims. (01. 250-215) My invention relates to electron discharge devices more particularly to improvements in phototubes.

The invention has for its object an improved 5 photo-cathode and method of manufacturing the same. a

According to the invention the silver layer which is to form the photocathode base and is to be oxidized entirely or partly and which is then to be submitted to the action of a photoelectric metal is formed by precipitation of silver particles introduced with such a concentration into an atmosphere of inert gas of sucha pressure, that the silver particles precipitated from this atmosphere form a non-reflective layer and are smaller than 0.5 micron, preferably of the order of magnitude of 0.01 micron. The silver particles may be introduced into thegaseous atmosphere, for example, by thermal vaporization or by disintegration with the aid of an electric discharge in which the silver may constitute the cathode.

It has been found that if the silver layer is treated in the known manner described, that is to say is oxidized and is submitted to the action of the photoelectric metal, a photo-electric electrode is obtained which possesses a high sensitiveness. The silver layer has a dull surface and a dark color. The color depends on the size of the silver particles by which the layer is built up. A layer of particles of about 0.01

micron has a dull-black color whereas with a size of the particles of 0.1 micron the color is dark gray.

The size of the particles depends on the concentration utilized in the precipitation from an atmosphere of inert gas. The higher the concentration, the larger are also the dimensions of the silver particles precipitated. In the case of thermal vaporization of the silver the speed of vaporization may be increased by the use of higher temperatures. If the silver particles are introduced into the gaseous atmosphere by disintegration, the amount of silver introduced into the gaseous atmosphere per unit of time may be increased by increasing the current intensity and by intensifying the electric field.

The pressure of the gaseous atmosphere also I acts upon the size of the silver particles precipitated. Since in the case of very low gas pressures silver precipitates atomically, owing to which, as in the case of vaporization in a high vacuum, a completely coherent silver layer having a reflecting surface is obtained, the pressure of the gas must be taken sufliciently high to avoid the production of such a reflective layer. 0n the other hand, the gas pressure should not be taken too high for an excessive pressure greatly decreases the speed at which the silver particles may be introduced into the gaseous atmosphere. 6 In general, gas pressures ranging from 0.05 to 5 mm. appear best.

The concentration of the silver particles in the gaseous-atmosphere as well as the pressure of the latter are so chosen thatthe silver atoms com- 10 bine with each other prior to the precipitation (consequently no precipitationlof atomic silver) but form only particles of very, small dimensions (smaller than 0.5micron) The smaller the particles by which the silver layer is built up the w darker is the color of the layer. If the particles have dimensions of about 0.01 micron, the layer produced has, as mentioned hereinbeiore, a deepblack color. If the particles become still smaller, the reflecting capacity may return. As it has 20 been found that reduction of the dimensions of the constituent particles results in a higher sensitiveness of the photo-electric electrode, these particles are preferably made materially smaller than 0.5 micron, for example smaller than 0.1 25 micron, and preferably even of the order of magnitude of 0.01 micron.

The most desirable gas pressure and the most advantageous concentration of the silver particles in the gaseous atmosphere may be experi- 30 mentally determined for any particular case. The concentration and the gas pressure to be chosen also depend to some extent on the distance of the silver introduced into the gaseous atmosphere in the form of particles from the spot at which the silver layer is to be formed. With a large distance the silver atoms have better opportunity to combine with each other so that with the same gas pressure and the same speed of vaporization particles of larger dimensions are precipitated than in the case of a. smaller distance. With too small a distance, however, the atoms would have no opportunity at all to combine with each other and the silver would precipitate atomically and 45 form a reflective surface.

For the gasfilling into which the silver particles are introduced, are utilized gases which are inert relatively to the silver particles that is to say which do not form a compound with said 50 particles, such, for example, as nitrogen, argon or some other rare gas.

The invention will be explained more fully with reference to the accompanying drawing which represents diagrammatically, by way of exam 55 pie, 9. photo-electric tube having an electrode tially spherical shape and has, for example, a

diameter of 4 ems.

This tube may be manufactured as follows: The filament 4, consisting, for example, of tungsten or molybdenum and arranged in the neighborhood of the center of the spherical tube, is coated with about 40 milligrams of silver prior to its mounting in the tube. After the tube has been completely exhausted about half the silver is vaporized; the silver vaporized deposits on the inside of the wall of the tube and forms there a silver layer 5 which makes a satisfactory contact with the current supply wire 2. Subsequently, a quantity of argon at a pressure of 1.7 ms. of mercury is introduced into the tube, and a current of such intensity is sent through the filament 4 that the silver still present thereon vaporizes within ten seconds. The silver vaporized precipitates on the silver layer 5 in the form of a non-reflective layer B. A screen I prevents the stem of the tube from being covered with a deposit of silver. A window 8 is also provided in a conventional manner by shielding or gently heating after depositing the silver-layer.

After the formation of the silver layer 6 the argon is removed from the tube by means of a pump and oxygen is admitted into the tube, preferably at a pressure of 0.15 mm. of mercury. In this atmosphere of oxygen there is produced an electric discharge during which the silver layers 5 and 6 act as the cathode and the wire 4 as the anode. The silver layer 6 and as the case may be, part of the layer 5 are oxidized due to said discharge. The degree of said oxidation may be regulated with the aidof the intensity of the discharge current and of the duration of the discharge. After a suflicient degree of oxidation of the silver has been attained, the r :cess of oxygen is removed from the envelope into which subsequently a quantity of caesium is introduced, which may be effected by distilling the caesium over into the tube or by liberating it within the tube by heating a pastii consisting of a caesium compound with a reducing agent. The tube is then heated to about 180 C. while not connected to a'yacuum'pump. The caesium reduces the silver oxide with the result that a layer of a mixture of caesium oxide and silver particles is obtained. Besides, some free caesium penetrates into said layer while the latter adsorbs a quantity of caesium.- After the formation of this electrode the excess of caesium may be removed, for example, by heating the stem of lead glass.

The photo-electric electrode thus manufactured possesses a very high sensitiveness. For illustration it may be noted that the average sensitiveness of a number of electrodes manufactured in the manner described amounted to micro-amperes/lumen whereas the averages sensitiveness of a number of electrodes manufactured in the samemanner but with the difference that the oxidized silver layer was not obtained by vaporization in a gaseous atmosphere but wholly by vaporization in a vacuum, amounted to 40 micro-amperes/lumen. v

If desired, the tube described may also be providedwith a gasfllling. After the manufacture of the photo-electric electrode, for example, some metal.

argon may be introduced for this purpose into the tube at a pressure of 0.1 mm. of mercury. Alternatively, the gasfllling may be provided after the silver layer 6 has been oxidized and before the caesium is liberated within the tube or before, subsequent to the introduction of the caesium, the tube is submitted to the thermal treatment.

When employed in a gasfiiled photo-electric tube, the photo-electric electrode manufactured in the above described manner affords particular advantages, for it has been found that this electrode affords not only the advantage that the primary electron emission, that is to say the number of electrons emitted owing to the irradiation with a determined quantity of light, is large, but also the additional advantage that any positive ion formed in the gasfilling and impinging on the photo-electric electrode, liberates from the latter comparatively few electrons, which results in that the tube breaks down less rapidly, that is .to say that the danger of the production of a glow discharge which can no longer be regulated by the exposure is reduced while in addition the photo-electric current has a smaller inertia.

' It is evident that the silver layer manufactured in accordance with the invention may also be applied directly to glass or to another insulating support. It is also possible, of course, to apply the silver layer to a separate metallic plate mounted within the tube or to a metal layer provided on the glass wall and formed in a manner different fromthat above described, for example, by precipitation from a solution.

While I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only one specific application for which my invention may be employed,

it will be apparent that my invention is by no.

means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

What I claim as new is:

l. The method ofmanufacturing a photo-electric cathode having a non-reflective silver layer composed of'siiver particles smaller than .5 micron including the steps of precipitating a silver layer on a base in the presence of an inert gas, oxidizing said silver layer and reducing the oxidized silver layerby means of a photo-electric 2. The method of manufacturing a photoelectric cathode having a non-reflective silver layer composed of silver particles smaller than .5 micron including the steps of precipitating silver layer on a base in'an evacuated envelope, introducing an inert gas into the envelope and further precipitating silver particles on said layer in the presence of an inert gas, oxidizing the resulting silver layer, and reducing the silver oxide layer by means of a photo-electric metal.

3. The method of manufacturing a photoelectric cathode having a non-reflective silver layer and composed of particles smaller than .5

micron including the steps of introducing a quantity of silver within an evacuated envelope vaporizing and precipitating approximately half of said silver on a base withinsaid envelope, ad-

mitting argon at a pressure of substantially 1.7

millimeters within said envelope and vaporizing the remainder of said silver in the presence of said argon and precipitating the same upon saidbase, removing said argon, oxidizlng said precipitated silver and admitting caesiumto said envelope, reducing the oxidized silver by' said caesium in the presence of heat at about 180*. C.

and removing the excess caesium by means of a caesium absorbingelement.

4. A phototube including an evacuated enve- -lope, an anode and a photocathode within said MAR'I'EN CORNELIS TEVES. 

